Generally, a projection display device lens is dust-proof and sheltered from light or protected from impact mainly by means of a projection lens cap. Sometimes when a user wants to pause the projection of the projection display device and proceeds with other illustration or to write, meanwhile, without switching on the projection display device so frequently that the lifetime thereof will be shortened, a projection lens cap is used to cover the projection lens for intercepting the projection temporarily. At the same time, because the projection display device is still at a projecting mode, the light beam with great heat projected from the projection lens keeps being projected on the projection lens cap and results in heat accumulated on the projection lens cap, which causes the projection lens cap made of plastic material to melt. Please refer to FIG. 1, which shows a conventional projection display device 10. The light beam with great heat is reflected back to the projection lens 11 mainly by pasting a light-reflecting sheet or a metal blocking sheet on one side of the projection lens cap 12 opposite to the projection lens 11. However, even though the problem of heat accumulated on the projection lens cap 12 is solved, heat accumulates on the projection lens 11 instead and damages the projection lens 11, which causes the quality of images projected from the projection lens 11 to be lowered. Another conventional way is to select a plastic material with a high heat-resistant coefficient to make the projection lens cap 12, however, this not only increases the cost but also needs to select a plastic material with a different heat-resistant coefficient again with the increase of brightness, which is quite inconvenient.